I've always thought The Drode would have made a better nemesis for Marco instead of Rachel. I would have loved s book where they try to put it each other
Interesting. You're right that Marco has that almost excessive concern for order. Like, he claims to be chaotic, but a few pranks in a classroom do not a chaotic character make. He enforces Jake's leadership more than any other character, he turns the order structures of the Yeerk Empire to his advantage, and he understands the importance of learning the rules so that he can fuck with those rules. Having him be countered by an agent of chaos has some interesting potential.
That said, I do like that his primary nemesis is Visser "I make the rules work for me" One, and that he's the only one who wins a major victory in the war through politics rather than force. To me, Rachel fits as the counter to Drode because she's defined (and helped, and threatened by) her chaotic nature. She finds the crazy solution, she cuts the Gordian knot rather than trying to unpick it, she wins victory by simply damning the social norms and running at the problem with a knife, and she almost gets her friends killed with her refusal to follow orders. Drode feels like a natural counter to her.
Like, the Freudian Masterplot roughly means "this minor character exists to show what would happen if the protagonist gave into their worst impulses." (Classic example: how Ozai represents a possible future for Zuko.) To me each Animorph's nemesis fits that. Visser One is a version of Marco who chooses politics and family above everything. Visser Three is a version of Ax who takes andalite superiority to bonkers extremes. Drode is Rachel if she starts to value freedom and chaos above connection. Taylor is Tobias's misanthropy and alienation turned toxic. Tom's yeerk has Jake's "I look harmless" vibe and Jake's "my temper will kill everything I love" weakness. Aldrea* is what would happen if Cassie let her concern for others slip into condescension. IMHO, I like Drode in there.